Optical disc device which perform recording and playback of information with respect to CD, DVD, and the like which are optical recording mediums for the information on music and videos, using laser light, such as linearly polarized light and circularly polarized light, is widely used. As an optical pickup device among them, which is compatible with CD and DVD, is popularized, there is a growing demand for miniaturization of the device, and attempts have been made to miniaturize the optical pickup device by simplification, such as reducing the number of optical elements.
A DVD is so designed as to be able to retain video and audio information of over two hours in a single disc, and has a higher recording density than a CD, so that the playback wavelength of the DVD becomes shorter to 655 nm as compared with 785 nm for the CD. An optical pickup device compatible with the DVD and CD inevitably requires two different wavelengths, two laser light sources to cope with the two wavelengths, and respective sets of optical elements, such as wave plates. As a result, the optical pickup device is composed of two pickups. With the recent demand for miniaturization of the optical pickup device, however, various attempts have been made to construct the device with a single pickup.
Polarized light used in an optical pickup will be described below. Light is one of waves which is called an electromagnetic wave. A plane which includes the progressing direction of light and a magnetic field vector is called a plane of polarization. A plane which includes the progressing direction of light and an electric field vector is called a plane of vibration. Light in which the directions of planes of polarization are the same is called polarized light. Further, polarized light whose plane of polarization is limited to a single plane is called linearly polarized light that includes P-polarized light, a component which vibrates horizontally with respect to a plane including an incident light and line normal to an incident plane, and S-polarized light, a component which vibrates vertically with respect to a plane including the incident light and line normal to the incident plane.
Polarized light whose electric field vector rotates with the passage of time as seen at a given position is generally called elliptically polarized light, and particularly, when the distal end of electric field vector is projected on a plane perpendicular to the progressing direction of light, the one whose locus is a circle is called circularly polarized light.
FIG. 14 is a diagram showing a zero-order wave plate 3 which is constructed by laminating a first multiple-order wave plate 1 (thickness d1) with a phase difference δ1 (2790°) and a second multiple-order wave plate 2 (thickness d2) with a phase difference δ2 (2700°) in such a way that the crystal optical axes cross each other at 90°, and functions as a quarter-wave plate. FIG. 14(a) is a diagram showing the angle of intersection of crystal optical axes 4 and 5 of the first and second wave plates 1 and 2 as seen from the incident plane of the wave plate 3, and FIG. 14(b) is a perspective view showing the configuration of the wave plate 3.
This can cancel out an extra phase difference by setting the angle of intersection of the crystal optical axes to 90°, i.e., δ1 −δ2=2790°−2700°=90°, and functions as a zero-order quarter-wave plate. Therefore, as the linearly polarized light 6 enters the wave plate 3, the phase is shifted by 90° at the emergence plane so that it is output as circularly polarized light 7.
The phase difference δ3 of the wave plate 3 can also be given by the following formula.δ3=δ1−δ2=2π×Δn×(d1−d2)/λ  (1)wherein: Δn is a refractive index difference between the first and second wave plates 1 and 2, and λ is the wavelength of incident light.
FIG. 15 is a perspective view showing a zero-order wave plate 8 (thickness d3) which functions as a zero-order quarter-wave plate with a phase difference δ4 (=90°). As a linearly polarized light 9 enters the wave plate 8, the phase is shifted by 90° at the emergence plane so that it is output as circularly polarized light 10.
The phase difference δ4 of the wave plate 8 can be given by the following formula.δ4 =2π×Δn×d3/λ  (2)wherein: Δn is a refractive index difference (Ne−No) of the wave plate 8, λ is the wavelength of incident light, No is the refractive index of ordinary ray, and Ne is the refractive index of extraordinary ray.
The following problem arises when an attempt is made to construct a 2 different wavelength optical pickup device with a single pickup by adequately selecting those wave plates 3 and 8 and arranging them at predetermined positions in the pickup.
When a pickup is constructed in such a way that the single quarter-wave plate 3 for playback of a CD (785 nm) as shown in FIG. 16 copes with two different wavelengths in order to reduce the number of elements to miniaturize the optical pickup device as mentioned above, as shown in FIG. 16(a), as P-polarized light 11 enters a light splitter 12 (hereinafter, “PBS”), it passes a mirror 13 which is formed by an optical thin film having characteristics to pass P-polarized light and reflect S-polarized light, and enters the quarter-wave plate 3 as P-polarized light. As the phase is shifted by 90° here, the light is output as circularly polarized light 14 and is input to a pit 15 of the CD. At the time the circularly polarized light 14 is reflected at the pit 15, it is reflected as circularly polarized light 16 with the opposite rotational direction, so that when the circularly polarized light 16 enters the quarter-wave plate 3, it is output as S-polarized light, is reflected at the mirror 13 of the PBS 12 and reaches a photodetector (hereinafter, “PD”) not shown, thereby ensuring the use of the laser light with efficiency of 90% or more. For the sake of easier description, the optical axis is shifted between a forward path and a return path in FIG. 16.
In a case of playing back a DVD, on the other hand, as shown in FIG. 16(b), when P-polarized light 11 with a wavelength of 655 nm enters the PBS 12, it passes the mirror 13 and enters a quarter-wave plate 3 as the P-polarized light. At this time, the quarter-wave plate 3 has a function of shifting the phase by 90° C. only with respect to a single wavelength of 785 nm, so that conversion from linearly polarized light to circularly polarized light cannot be carried out sufficiently and the light is output as elliptically polarized light 17. When this light enters the pit 15 of the DVD, it is reflected as elliptically polarized light 18 whose rotational direction is opposite to that of the elliptically polarized light 17 and is input to the quarter-wave plate 3. Likewise, it cannot be sufficiently converted to linearly polarized light, i.e., it is output from the quarter-wave plate 3 in a state where an elliptically polarized light component and an S-polarized light component are mixed, and the S-polarized light component alone is reflected at the mirror 13 of the PBS 12 while the elliptically polarized light component passes the mirror 13. Therefore, from the results of the experiments conducted by the present inventor, for example, about 65% of laser light is detected by a PD from the viewpoint of the optical efficiency, and remaining about 30% is lost as an elliptically polarized light component which passes the mirror, raising a problem in view of efficiency. This is also seen from the formulas (1) and (2) representing the phase differences of the wave plates 3 and 8 and showing that the phase differences depend on the wavelength.
Japanese Patent No. 3174367 proposes a broadband quarter-wave plate wherein a laminated wave plate, constructed by laminating an extension film having a phase difference of a half wavelength (180°) with respect to monochromatic light and an extension film having a phase difference of a quarter wavelength (90°) in such a way that those crystal optical axes cross each other, has a function of shifting the phase by90° in a wide band. When the broadband quarter-wave plate is used in an optical pickup device which records and plays back a DVD (655 nm) and CD (785 nm), a single wave plate copes with two wavelengths, so that the demand of simplifying pickups to nearly a single one can be satisfied.
As shown in FIG. 17, FIG. 5 of Japanese Patent No. 3174367 discloses a graph of the wavelength dependence of transmittance obtained by evaluating the spectrum with the broadband quarter-wave plate placed between polarization plates laid out in the crossed nicols arrangement.
Paying attention to the curve of Example 3 in the graph or the transmittance of the broadband quarter-wave plate, however, the transmittance gradually increases from 40% to 50% as the wavelength moves from 400 nm to 800 nm, i.e., the graph has an inclined characteristic. It is apparent that the efficiency of the function as a quarter-wave plate changes according to the wavelength. It is the transmittance of 50% at which the wave plate completely functions as a quarter-wave plate. That is, this broadband quarter-wave plate has not solved the wavelength dependence completely and has different efficiencies of shifting the phase by 90° depending on the wavelengths, and therefore has a problem that it cannot fulfill the strict specifications on the optical characteristics which are demanded of a wave plate from a viewpoint of the optical efficiency or the like in a recent optical pickup device which is compatible with a DVD/CD.
The present invention has been achieved in order to overcome the above problem and aims at providing a wave plate that completely functions as a quarter-wave plate with respect to a plurality of wavelengths in an optical pickup device or the like which is compatible with a DVD/CD, and an optical pickup using the wave plate.